Single-side microelectromechanical capacitive acclerometer and method of making same

ABSTRACT

A high sensitivity, Z-axis, capacitive microaccelerometer having stiff sense/feedback electrodes and a method of its manufacture on a single-side of a semiconductor wafer are provided. The microaccelerometer is manufactured out of a single silicon wafer and has a silicon-wafer-thick proof mass, small and controllable damping, large capacitance variation and can be operated in a force-rebalanced control loop. One of the electrodes moves with the proof mass relative to the other electrode which is fixed. The multiple, stiffened electrodes have embedded therein damping holes to facilitate force-rebalanced operation of the device and to control the damping factor. Using the whole silicon wafer to form the thick large proof mass and using thin sacrificial layers to form narrow uniform capacitor air gaps over large areas provide large capacitance sensitivity. The manufacturing process is simple and thus results in low cost and high yield manufacturing. In one preferred embodiment, the fixed electrode includes a plurality of co-planar, electrically-isolated, conductive electrodes formed by thin polysilicon deposition with embedded vertical stiffeners. The vertical stiffeners are formed by refilling vertical trenches in the proof mass and are used to make the fixed electrode stiff in the sense direction (i.e. Z or input axis). The moving electrode is dimensioned and supported for movement on the proof mass so as to be stiff in the sense direction. In another embodiment both of the electrodes are electroplated. In yet another embodiment four support beams support the proof mass at an upper portion thereof and four support beams support the proof mass at a lower portion thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This applications is a divisional of application Ser. No. 09/386,656filed on Aug. 31, 1999, now U.S. Pat. No. 6,167,757, which is acontinuation-in-part of U.S. application Ser. No. 08/925,257 filed Sep.8, 1997, now U.S. Pat. No. 6,035,714, entitled “MicroelectromechanicalCapacitive Accelerometer And Method Of Making Same”.

This application also claims benefit of U.S. provisional applicationSerial No. 60/111,370, filed Dec. 8, 1998, entitled “High SensitivityCapacitive Microaccelerometer With A Folded- Electrode Structure”.

GOVERNMENT RIGHTS

This invention was made with government support under Contract Nos.DABT63-95-C-0111 and F30602-98-2-0231, awarded by the Defense AdvancedResearch Projects Agency (DARPA). The government has certain rights inthe invention.

TECHNICAL FIELD

This invention relates to capacitive accelerometers and, in particular,to single-side microelectromechanical capacitive accelerometers andmethods of making same.

BACKGROUND ART

The constant drive toward small size, lightweight, low cost andlow-power sensing systems in all application domains has made highsensitivity and precision microaccelerometers increasingly needed.

These accelerometers are used in numerous applications, such as inertialnavigation and guidance, space microgravity measurements, seismology andplatform stabilization. Also, as they become manufacturable at low costwith small size, they attain a large potential consumer market in theirapplication as a GPS-aid to obtain position information when the GPSreceivers lose their line-of-sight with their satellites.

High precision accelerometers are typically operated closed-loop tosatisfy dynamic range, linearity and bandwidth requirements, in additionto high sensitivity and low-noise floor.

Capacitive microaccelerometers are more suitable since they providestable DC-characteristics and high bandwidth. Such accelerometers may befabricated by surface micromachining or bulk micromachining. The surfacemicromachined devices are fabricated on a single silicon wafer. However,they generally have low sensitivity and large noise floor, and thuscannot satisfy the requirements of many precision applications.

Some high resolution accelerometers are bulk micromachined and usemultiple wafer bonding as part of their manufacturing process. Thiswafer bonding is a complex fabrication step, and hence results in loweryield and higher cost. Also, forming damping holes in the thick bondedwafers is difficult, and thus special packaging at a specified ambientpressure is typically needed to control the device damping factor.Finally, due to wafer bonding, these devices show higher temperaturesensitivity and larger drift especially if glass wafers are used.

The above-noted patent application entitled “MicroelectromechanicalCapacitive Accelerometer And Method Of Making Same” utilizes a singlewafer fabrication technology with damping holes. However, fabrication ofthe accelerometer requires double side processing and lead transfer fromboth sides of the wafer. As shown in FIG. 1, the accelerometer,generally indicated at 10, includes a proof mass 12 suspended bycompliant beams 14 between two fixed and rigid electrodes 16. In thepresence of an external acceleration, the proof mass 12 moves from itscenter position and thus C_(S1) and C_(S2) change in oppositedirections. The proof mass 12 is rebalanced to its center position byapplying an electrostatic force to either the top electrode 16 or thebottom electrode 16.

U.S. Pat. No. 5,345,824 discusses a monolithic capacitive accelerometerwith its signal conditioning circuit fabricated using polysilicon proofmass and surface micromachining.

U.S. Pat. No. 5,404,749 discusses a boron-doped silicon accelerometersensing element suspended between two conductive layers deposited on twosupporting dielectric layers.

U.S. Pat. No. 5,445,006 discusses a self-testable microaccelerometerwith a capacitive element for applying a test signal and piezoresistivesense elements.

U.S. Pat. No. 5,461,917 discusses a silicon accelerometer made of threesilicon plates.

U.S. Pat. No. 5,503,285 discusses a method for forming anelectrostatically force rebalanced capacitive silicon accelerometer. Themethod uses oxygen implantation of the proof mass to form a buried oxidelayer and bonding of two complementary proof mass layers together. Theimplanted oxide layer is removed after bonding to form an air gap andrelease the proof mass.

U.S. Pat. No. 5,535,626 discusses a capacitive microsensor formed ofthree silicon layers bonded together. There is glass layer used betweeneach two bonded silicon pairs.

U.S. Pat. No. 5,540,095 discusses a monolithic capacitive accelerometerintegrated with its signal conditioning circuitry. The sensor comprisestwo differential sense capacitors.

U.S. Pat. No. 5,559,290 discusses a capacitive accelerometer formed ofthree silicon plates, attached together using a thermal oxide interface.

U.S. Pat. No. 5,563,343 discusses a lateral accelerometer fabricated ofa single crystal silicon wafer.

U.S. Pat. No. 5,605,598 discloses a monolithic micromechanical vibratingbeam accelerometer having a trimmable resonant frequency and method ofmaking same.

U.S. Pat. Nos. 5,594,171 and 5,830,777 disclose capacitance-typeacceleration sensors and methods for manufacturing the sensors. Thesensors include a mass portion having a plurality of movable electrodes.The sensors also include a plurality of stationary electrodes. Thesensors are manufactured on a single-side of a substrate.

U.S. Pat. No. 5,665,915 discloses a semiconductor capacitiveacceleration sensor. The construction of the sensor includes a basesubstrate having a first electrode attached to the top of the substrate.The sensor also includes a second electrode positioned between thesubstrate and the first electrode. The first electrode is a stationaryelectrode and the second electrode is a movable electrode.

U.S. Pat. No. 5,719,336 discloses a capacitive acceleration sensorhaving a first fixed electrode, a second fixed electrode, a firstmovable electrode, and a second movable electrode. The stationaryelectrodes are positioned in a configuration surrounding the movableelectrodes.

U.S. Pat. Nos. 5,392,651; 5,427,975; 5,561,248; 5,616,844; and 5,719,069disclose various configurations of microminiature accelerometers havingboth stationary and movable electrodes, wherein the electrodes arearranged in various configurations.

The paper entitled “Advanced Micromachined Condenser Hydrophone” by J.Bernstein et al, Solid-State Sensor and Actuator Workshop, Hilton Head,South Carolina, June, 1994, discloses a small micromechanical hydrophonehaving capacitor detection. The hydrophone includes a fluid-filledvariable capacitor fabricated on a monolithic silicon chip.

The paper entitled “Low-Noise MEMS Vibration Sensor for GeophysicalApplications” by J. Bernstein et al., DIGEST OF HILTON-HEAD SOLID STATESENSOR AND ACTUATOR WORKSHOP, pp. 55-58, June, 1998, discloses anaccelerometer having trenches etched in its proof mass to reduce dampingand noise floor.

The paper entitled “High Density Vertical Comb Array MicroactuatorsFabricated Using a Novel Bulk/Poly-Silicon Trench Refill Technology”, byA. Selvakumar et al., Hilton Head, S.C., June 1994, discloses afabrication technology which combines bulk and surface micromachiningtechniques. Trenches are etched and then completely refilled.

Numerous U.S. patents disclose electroplated microsensors such as U.S.Pat. Nos. 5,216,490; 5,595,940; 5,573,679; and 4,598,585.

Numerous U.S. patents disclose accelerometers such as U.S. Pat. Nos.4,483,194 and 4,922,756.

U.S. Pat. No. 5,146,435 discloses an acoustic transducer including aperforated plate, a movable capacitor plate and a spring mechanism, allof which form a uniform monolithic structure from a silicon wafer.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a single-side,microelectromechanical capacitive accelerometer including a fixedelectrode suspended between a proof mass and a moving electrode toprovide differential capacitance measurement and force-rebalancing.

Another object of the present invention is to provide a single-side,microelectromechanical capacitive accelerometer formed from a singlewafer with a proof mass having a thickness substantially equal to thethickness of the wafer, controllable/small damping and large capacitancevariation.

Yet another object of the present invention is to provide a single-side,microelectromechanical capacitive accelerometer wherein packaging andpotential integration of the device with its CMOS interface circuitry issimplified since all interconnect leads are on one side of the wafer.

In carrying out the above objects and other objects of the presentinvention, a microelectromechanical capacitive accelerometermanufactured on a single-side of a semiconductor wafer is provided. Theaccelerometer includes a fixed electrode, a movable proof mass having atop surface and a movable electrode. The movable electrode is attachedto the top surface of the proof mass to move therewith. The fixedelectrode is suspended between the proof mass and the movable electrode.

The accelerometer has an input axis and, preferably, both of theelectrodes are sufficiently stiff to electrostatically force-balanceproof mass displacement due to acceleration along the input axis withoutsubstantial bending of the electrodes along the input axis.

Each of the electrodes includes a planar layer which is relatively thinalong the input axis and, preferably, at least one of the planar layersis dimensioned and is formed of a material so that its electrode isstiff along the input axis.

Each of the electrodes may include an electroplated planar layer.

The accelerometer may include upper and lower beams for suspending theproof mass in spaced relationship from the fixed electrode.

Preferably, the fixed electrode includes a plurality of stiffenersextending from its planar layer along the input axis to stiffen thefixed electrode. The stiffeners extend toward the proof mass from theirplanar layer. The proof mass includes a plurality of cavities on its topsurface. The stiffeners are received within the cavities. The stiffenersand the proof mass have a substantially uniform, narrow air gaptherebetween.

The planar layer and the stiffeners are preferably formed of differentforms of the same material such as a semiconductor material likesilicon.

Also, preferably, each of the planar layers of the electrodes has aplurality of damping holes formed completely therethrough.

The proof mass is typically formed from a single silicon wafer having apredetermined thickness and wherein the thickness of the proof mass issubstantially equal to the predetermined thickness. At least one of theplanar layers and the proof mass are, preferably, formed of differentforms of semiconductor material.

Further in carrying out the above objects and other objects of thepresent invention, a single-side, microelectromechanical capacitiveaccelerometer having an input axis is provided. The accelerometerincludes first and second spaced conductive electrodes. Each of theconductive electrodes includes a planar layer which is relatively thinalong the input axis, but is stiff to resist bending movement along theinput axis. The accelerometer also includes a proof mass which isthicker than either of the planar layers by at least one order ofmagnitude along the input axis. The accelerometer further includes afirst support structure for supporting the proof mass in spacedrelationship from the first conductive electrode, and a second supportstructure for supporting the second conductive electrode on the proofmass. The second conductive electrode moves with but is electricallyisolated from the proof mass. The second conductive electrode and theproof mass move together in opposite directions relative to the firstconductive electrode. The conductive electrodes and the proof mass forma pair of substantially uniform, narrow air gaps on opposite sides ofthe first conductive electrode. The conductive electrodes and the proofmass form a pair of acceleration-sensitive capacitors.

Preferably, both of the conductive electrodes are sufficiently stiff toelectrostatically force-balance proof-mass displacement due toacceleration along the input axis without substantial bending of theconductive electrodes along the input axis.

At least one of the planar layers may be dimensioned and is formed of amaterial so that its conductive electrode is stiff along the input axis.

At least one of the planar layers may be an electroplated planar layer.

The first conductive electrode preferably includes a plurality ofstiffeners extending from its planar layer along the input axis tostiffen the first conductive electrode. The stiffeners extend towardsthe proof mass which includes a plurality of cavities which receive thestiffeners. The stiffeners and the proof mass have one of thesubstantially uniform, narrow air gaps therebetween. The planar layer ofthe first conductive electrode and the stiffeners are formed ofdifferent forms of the same material. Preferably, the material is asemiconductor material such as silicon.

The first conductive electrode preferably comprises a plurality ofco-planar, electrically isolated conductive electrodes.

Preferably, the proof mass is formed from a single silicon wafer havinga predetermined thickness. The thickness of the proof mass issubstantially equal to the predetermined thickness.

Also, preferably, the planar layer of at least one of the conductiveelectrodes has a plurality of damping holes formed completelytherethrough.

The first support structure preferably includes a plurality of beams forsuspending the proof mass at upper and lower portions of the proof mass.

Yet still further in carrying out the above objects and other objects ofthe present invention in a method for making a high-sensitivity,microelectromechanical capacitive accelerometer including a proof masshaving a thickness along an input axis of the accelerometer and firstand second conductive electrode from a single semiconductor wafer havinga predetermined thickness, an improvement is provided. The improvementincludes the steps of depositing first and second planar layers on asingle-side of the wafer. The planar layers are relatively thin alongthe input axis. The method also includes the step of stiffening thefirst and second planar layers to form the first and second conductiveelectrodes, respectively, which are stiff so as to resist bendingmovement along the input axis. The method then includes the step offorming substantially uniform first and second narrow gaps between thefirst conductive electrode and the proof mass and between the secondconductive electrode and the first conductive electrode, respectively.The thickness of the proof mass is at least one order of magnitudegreater than either the thickness of the first planar layer or thethickness of the second planar layer.

Preferably, the thickness of the proof mass is substantially equal tothe predetermined thickness of the wafer and the semiconductor wafer isa silicon wafer.

The step of stiffening may include the step of forming a stiffeningmetallic layer on at least one of the planar layers.

The step of stiffening may include the step of forming stiffening ribson at least one of the planar layers.

The step of forming the stiffening ribs, preferably, includes the stepsof forming trenches in the proof mass and refilling the trenches with asacrificial layer having a substantially uniform thickness and anelectrode material. The step of forming the substantially uniform, firstnarrow air gap then includes the step of removing the sacrificial layer.

The step of stiffening may include the step of electroplating the firstand second planar layers.

The method may also include the step of forming a plurality of beams forsupporting the proof mass at upper and lower portions of the proof mass.

Several significant innovative features of the accelerometer structureand its manufacturing technique include: 1) forming both fixed andmoving sense/feedback electrodes with embedded damping holes usingstiffened deposited polysilicon layers or electroplated metal layers; 2)forming electrically-isolated stand offs on a top surface of a thickproof mass for the moving sense/feedback electrode; 3) forming uniformnarrow gaps over a large area between the two electrodes and between thefixed electrode and the proof mass by etching sacrificial layers; and 4)using a single silicon wafer for manufacturing on a single-side thereofthe accelerometer without any need for wafer bonding.

The sensor typically is operated in a closed-loop mode. Preferably, aswitched-capacitor, sigma-delta modulator circuit is utilized toforce-rebalance the proof mass and provide direct digital output for theaccelerometer.

The above objects and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side schematic view of a prior art closed-loop, capacitiveaccelerometer;

FIG. 2 is a side schematic view of a single-side, closed-loop,microelectromechanical capacitive accelerometer constructed inaccordance with the present invention;

FIG. 3 is a schematic, perspective view, in cross-section, of anaccelerometer constructed in accordance with a first embodiment of thepresent invention with the thicknesses of the electrodes greatlyexaggerated for illustrative purposes;

FIG. 4 is a sectional view of the accelerometer of FIG. 3 taken alonglines 4—4;

FIG. 5 is a top plan view, partially broken away, of the accelerometerof FIG. 3;

FIG. 6 is a view similar to the view of FIG. 3 of a second embodiment ofthe invention;

FIG. 7 is a sectional view of the accelerometer of FIG. 6;

FIG. 8 is a view similar to the view of FIG. 5 of the second embodiment;

FIG. 9 is a view similar to the views of FIGS. 3 and 6 of a thirdembodiment of the invention;

FIG. 10 is a sectional view of the accelerometer of FIG. 9;

FIG. 11 is a graph of calculated, middle-electrode stiffness vs.vertical stiffener height for different plate lengths for the embodimentof FIGS. 3-5;

FIG. 12 is a graph of calculated, top-electrode stiffness vs. supportanchor length (plate width) for various anchor separations (platelength) for the embodiment of FIGS. 3-5; and

FIG. 13 is a graph of single-electrode, open-loop capacitance vs. inputacceleration for the embodiment of FIGS. 3-5.

BEST MODE FOR CARRYING OUT THE INVENTION

In general, an accelerometer, generally indicated at 20 in FIGS. 2-5 ofthe present invention, and its manufacturing technology address aresubstantially all high-precision, accelerometer design issues. Themethod of the present invention combines both surface and bulkmicromachining in order to achieve high device sensitivity, low noisefloor, and controllable damping—all by performing fabricating steps on asingle-side of a single silicon wafer. The central idea of the presentinvention is to perform fabricating steps to a single-side of a waferwhile at the same time utilizing the whole wafer thickness to attain alarge silicon proof mass 22 (i.e. typically about 450 microns thick fora four inch silicon wafer) together with top and middle stiffenedconductive electrodes 26 and 24, respectively, of the accelerometer 20.The middle stiffened conductive electrode 24 may comprise a plurality ofelectrically-isolated, fixed conductive electrodes 25 which overlay anisolation dielectric layer 27 at their opposite ends as illustrated inFIGS. 3-5. The top stiffened conductive electrode 26 is supported on andis electrically isolated from the proof mass 22 by standoffs, generallyindicated at 28 in FIGS. 3 and 4.

The electrodes 25 of the fixed electrode 24 are supported at theiropposite ends on a support rim 30 in fixed relationship to the movingelectrode 26 and the proof mass 22. The top electrode 26 is anchored toa top surface of the proof mass 22 so that it moves with the proof mass22. Therefore, the fixed electrode 24 is suspended between the movableproof mass 22 at its bottom and the moving electrode 26 at its top. Thispermits force rebalancing and differential capacitance sensing withoutthe need for sandwiching the thick proof mass 22 between two fixedelectrodes. Hence, the “folded-electrode” structure of FIG. 2 allowsrealization of accelerometers with thick, large proof mass and thinstiff electrodes on only one side of a silicon wafer. By incorporatingdamping holes 32 in the electrode 26 and damping holes 34 in theelectrodes 25 of the fixed electrode 24, squeeze film damping can becontrolled and reduced.

High-Sensitivity Accelerometer

FIGS. 3-4 show the structure of the high sensitivity accelerometer ordevice 20 with the “folded-electrode” structure (i.e. the top electrode26 is “folded” from the bottom surface of the proof mass 22 to its topsurface). It has high-capacitance sensitivity by using a whole waferthickness to obtain the large proof mass 22, and thin sacrificial layersduring the fabrication process to form uniform, narrow gaps over largeareas of the accelerometer. The device 20 is all-silicon and fabricatedon a single wafer by utilizing polysilicon electrodes 24 and 26 each ofwhich has a planar layer which is relatively thin.

The thin, middle electrode 24 is made rigid by embedding thick verticalstiffeners 36 in its planar layer as described in the above-notedutility patent application. The thick stiffeners 36 are formed by thinfilm deposition and refilling high aspect-ratio trenches 38 in the proofmass 22.

The top electrode 26 is made rigid by making it short and wide, andsupporting it through the electrically-isolated standoffs 28 on the topsurface of the proof mass 22. These standoffs 28 are formed by a firstpoly 39, and dielectric layers 40 at top and bottom surfaces of thefirst poly 39. Sacrificial oxide dielectric layers 41 between the proofmass 22 and first poly 39 are sealed by the poly dielectric layers 40and kept at the anchors 28 to bring the anchor height to the level of asecond poly which forms the electrodes 25.

In this manner, the curvature of the top electrode 26 at the anchors 28is reduced which helps to reduce spring softening effect at the anchors28. Also, anchor parasitic capacitances are reduced by keeping thelayers 41 of sacrificial oxide.

In order to verify the effectiveness of the geometrical design to formthe rigid electrodes 24 and 26 with thin film deposition, the stiffnessof the middle and top electrodes 24 and 26 for different geometries arecalculated and shown in FIGS. 6 and 7, respectively. These calculationsare based on mechanical design equations for uniformly loaded beams andplates since the electrodes 24 and 26 will be typically uniformly loadedby the electrostatic feedback force.

FIG. 11 shows the calculated middle electrode stiffness versus verticalstiffener height for different plate lengths for the embodiment of FIGS.3-5. The electrode thickness is 2.5 μm, and the width is 200 μm.

The calculated top electrode stiffness versus the support anchor length(plate width) for various anchor separation (plate length) is shown inFIG. 12 for the embodiments of FIGS. 3-5. The electrode thickness is 2μm and spring softening effects due to the non-straight plate shape nearthe anchors are not considered. As can be seen, a stiffness of severalhundred to a few thousand N/m's is achievable. For instance, anaccelerometer 20 fabricated with a 2.6 mm×1 mm proof mass 22 and nineisolated middle electrodes 25 has calculated middle and top electrodestiffnesses of 1240 [N/m] and 2000 [N/m], respectively. These are wellabove the minimum required stiffness of 200 [N/m] that electromechanicalsimulations of the closed-loop accelerometer 20 have indicated.

Referring again to FIGS. 3-5, accelerometer damping is reduced byforming the holes 34 in the electrodes 25 of the middle electrode 24.Hence, the device 20 can achieve a sub-μg mechanical noise floor atatmosphere without vacuum packaging. Metal pads 45 are formed atopposite ends of the electrodes 25.

Holes 32 created in the top electrode 26 provide access for sacrificialetchant and also help reduce the damping.

The proof mass 22 is suspended by polysilicon beams 42 supported on thesupport rim 30 by anchors 41 and by anchors 43 on the proof mass 22. Thebeams 42 also provide lead transfers from the top electrode 26 and theproof mass 22 to metal pads 44 on the anchors 41 supported on anisolation dielectric layer 47 on the rim 30. A full folded-beam orstraight beam bridge suspension configuration is used to obtain lowcross axis sensitivity.

Referring now to FIGS. 6-8, there is illustrated a second embodiment ofa microelectromechanical capacitive accelerometer, generally indicatedat 20′. Items shown in FIGS. 6-8 which are substantially the same instructure and function as those items shown in FIGS. 3-5 have the samereference numeral but have a single prime designation.

Referring now to FIGS. 9 and 10, there is illustrated a third embodimentof a microelectromechanical capacitive accelerometer, generallyindicated at 20″. Items shown in FIGS. 9 and 10 which are substantiallythe same in structure and invention as those items shown in FIGS. 3-5have the same reference numeral but have a double prime designation.

Referring again to FIGS. 6-8, the accelerometer 20′ has thickelectroplated metal sense/feedback electrodes 25′ and 26′. Theaccelerometer 20′ also has a silicon proof mass 22′ and suspension beams42′. The electrodes 25′ and 26′ are made stiff in the sense direction byusing a thick electroplated layer, as shown in FIGS. 6-8 which are theperspective, cross-sectional and top views, respectively, of theaccelerometer 20′. The electroplating method of manufacturing allows athick stiff plate formation without any need for embedded verticalstiffeners of the first embodiment of FIGS. 3-5. Moreover, theelectroplating fabrication process is performed at a low temperatureprocess which is compatible to be a post-process for microelectroniccircuitry. Hence, a full-monolithic, high-precision microaccelerometerwith interface circuitry can be manufactured at low cost and in a largevolume.

Referring again to FIGS. 9 and 10, there is illustrated an improvedaccelerometer 20″ which includes four additional support beams 50 at thebottom of proof mass 22″. This structure provides an almost symmetricalproof mass support and results in lower accelerometer cross-axissensitivity. The additional support beams 50 can be preferably formed byshallow boron diffusion at the first masking step of fabricationprocess. As described herein, in this step both sides of the siliconwafer are patterned and aligned together, and the bottom support beampatterns can be also included without any additional processing.

Fabrication Process

The fabrication process for the embodiment of FIGS. 3-5 consists oftwelve masking steps on the frontside and a masking step on the backsideof a single silicon wafer for the proof mass release. The process startswith a shallow p++ boron diffusion on <100> double-polished p-type Siwafer, using thermal oxide as a mask. Front and backside of the waferare patterned with different masks and the patterns are aligned to eachother. The shallow boron diffusion defines the proof mass 22 and thesupporting rim 30.

Then, a LPCVD nitride O₂ layer is deposited and patterned to form thefirst poly electrode anchors 28 and isolation dielectric under themiddle poly electrode dimples. The next masking step is etching trenches38 to define the middle electrode vertical stiffeners 36.

The trenches 38 are then refilled completely using sacrificial LPCVDoxide and LPCVD polysilicon. Two patterning steps are performed on thesacrificial oxide prior to the polysilicon deposition to form dimplesinside die middle electrodes 25 and the anchors on the rim 20 for themiddle electrode 24 and the top poly anchors 28 on the proof mass 22.The dimples reduce the contact area and help to avoid stiction. Thepolysilicon film is patterned to form the middle electrodes 25 and theirdamping holes 34, in addition to the supports 28 for the top poly.

Next, LPCVD nitride is deposited and patterned to form the second polyelectrode anchors 43 and isolation dielectric under the top polyelectrode dimples. Then, sacrificial LPCVD oxide is deposited to definethe air gap between top and middle electrodes 24 and 26, respectively.Similar to the patterning steps performed on the first sacrificialoxide, the deposited oxide layer is patterned in two consequent steps toform the dimples and anchors 28 for the top poly electrode 26. In fact,the mask layout is designed so that the latter three masking steps usethe same masks that are already used to form the first poly nitride andoxide anchors, and dimples.

A LPCVD polysilicon is then deposited and patterned to form the topelectrode 26, proof mass support, and top electrode lead transfer beams42. The top poly electrode 26 is sealed with thin LPCVD oxide in thenext step. This oxide is patterned to form metal contacts to polyelectrodes and silicon rim 30. The wafer is then patterned for Chromium(Cr)/Au (400 Å/5000 Å) sputtering and lift-off. In each masking step allthe deposited films on the backside is stripped before going to the nextstep. However, the last deposited oxide layer is patterned using thebackside mask, and the proof mass 22 is released from backside using acombination of Deep RIE and net silicon etch. Finally, the device iscompletely released after etching the sacrificial oxide in dilutedhydrofluric acid solution.

Fabrication & Measurement Results

A number of prototype microaccelerometers have been fabricated andtested. A device with a 2.6 mm×1 mm proof mass and 9 electricallyisolated middle electrodes 25 have been formed. The proof mass 22 had arectangular shape to produce a large sense capacitance area withoutmaking the middle electrodes 25 too long. Top and bottom air gapsuniformly separated the electrodes 24 and 26 from each other, and theproof mass 22 from the middle electrode 24 and its stiffeners 36.

A summary of device specifications and measured parameters is presentedin Table 1.

TABLE 1 Parameter Calculated Measured Proof mass Size 2.7 mgr 2600 ×1000 μm² Resonant Frequency 100 Hz — Suspension Beam 1.1 N/m — StiffnessMiddle Electrode 2400 N/m 1000 N/m Stiffness Top Electrode Stiffness1240 N/m 650 N/m Number of Electrodes — 9 Open-Loop Sensitivity 13.3pF/g 11.2 pF/g of a single Electrode Open-Loop Sensitivity 120 pF/g100.3 pF/g Mechanical Noise <0.18 μg/Hz —

The middle electrode stiffness was obtained by measuring its pull-involtage (19V) with the proof mass 22 held fixed by a probe tip.Similarly, the top electrode pull-in voltage is measured (16V) with themiddle electrode 24 and the proof mass 22 held in-place. The measuredstiffnesses of the electrodes 24 and 26 are less than the expectedcalculated values, which is mostly due to not considering the shapespring-softening effects and the polysilicon internal stress in thecalculations. Nevertheless, the measured stiffnesses of the middle andtop electrodes 24 and 26, respectively, are 2-3 orders of magnitude morerigid than the proof mass support beams 42 and above the properclosed-loop operation requirement.

The accelerometer open-loop sensitivity was measured electrostaticallyfor the embodiment of FIGS. 3-5. FIG. 13 shows the sensitivity versusinput acceleration for 1-of-9 electrodes. The input acceleration rangeis limited and hence a linear response is obtained indicating asensitivity of 11.1 pF/g per electrode—which is equivalent to asensitivity of 100.3 pF/g for the accelerometer 20. The mechanical noisefloor at atmospheric pressure is calculated to be 0.18 μg/Hz. Thiscalculation is based on considering only the squeeze film damping withthe damping holes 32 and 34. As can be observed, a sub-μg noise floorcan be achieved without a need for vacuum packaging.

SUMMARY

A high sensitivity capacitive silicon accelerometer 20 with a newfolded-electrode structure is described herein. The structure providesclosed-loop operation and differential capacitance measurement by usinga fixed rigid electrode 24 suspended between the proof mass 22 and astiff moving electrode 26. High device sensitivity is achieved byforming the thick silicon proof mass 22 and a narrow uniform air gapover a large area. The device 20 is all silicon and fabricated on oneside of a wafer. The electrodes 24 and 26 are formed by depositedpolysilicon, and are made rigid by embedded high aspect-ratio stiffeners36 or proper geometrical design. The mechanical noise floor is reducedby incorporating damping holes 32 and 34 in the electrodes, 26 and 24,respectively. The measured sensitivity for the device 20 with 2.6 mm×1mm proof mass 22 and 1.4 μm air gap is ≈100 pF/g. The calculatedmechanical noise floor for the same device 20 is 0.18 μg/Hz atatmosphere.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

What is claimed is:
 1. A single-side, microelectromechanical capacitiveaccelerometer having an input axis, the accelerometer comprising: firstand second spaced conductive electrodes, each of the conductiveelectrodes including a planar layer which is relatively thin along theinput axis, but is stiff to resist bending movement along the inputaxis; a proof mass which is thicker than either of the planar layers byat least one order of magnitude along the input axis; a first supportstructure for supporting the proof mass in spaced relationship from thefirst conductive electrode; and a second support structure forsupporting the second conductive electrode on the proof mass wherein thesecond conductive electrode moves with but is electrically isolated fromthe proof mass, and the second conductive electrode and the proof massmove together in opposite directions relative to the first conductiveelectrode, and wherein the conductive electrodes and the proof mass forma pair of substantially uniform, narrow air gaps on opposite sides ofthe first conductive electrode, and wherein the conductive electrodesand the proof mass form a pair of acceleration-sensitive capacitors. 2.The accelerometer as claimed in claim 1 wherein both of the conductiveelectrodes are sufficiently stiff to electrostatically force-balanceproof-mass displacement due to acceleration along the input axis withoutsubstantial bending of the conductive electrodes along the input axis.3. The accelerometer as claimed in claim 1 wherein at least one of theplanar layers is dimensioned and is formed of a material so that itsconductive electrode is stiff along the input axis.
 4. The accelerometeras claimed in claim 1 wherein at least one of the planar layers is anelectroplated planar layer.
 5. The accelerometer as claimed in claim 1wherein the first conductive electrode includes a plurality ofstiffeners extending from its planar layer along the input axis tostiffen the first conductive electrode.
 6. The accelerometer as claimedin claim 1 wherein the first conductive electrode comprises a pluralityof co-planar, electrically-isolated, conductive electrodes.
 7. Theaccelerometer as claimed in claim 1 wherein the proof mass is formedfrom a single silicon wafer having a predetermined thickness and whereinthe thickness of the proof mass is substantially equal to thepredetermined thickness.
 8. The accelerometer as claimed in claim 1wherein the planar layer of at least one of the conductive electrodeshas a plurality of damping holes formed completely therethrough.
 9. Theaccelerometer as claimed in claim 5 wherein the stiffeners extendtowards the proof mass from the planar layer of the first conductiveelectrode and wherein the proof mass includes a plurality of cavities,the stiffeners being received within the cavities and wherein thestiffeners and the proof mass have one of the substantially uniform,narrow air gaps therebetween.
 10. The accelerometer as claimed in claim5 wherein the planar layer of the first conductive electrode and thestiffeners are formed of different forms of the same material.
 11. Theaccelerometer as claimed in claim 10 wherein the material is asemiconductor material.
 12. The accelerometer as claimed in claim 11wherein the semiconductor material is a silicon material.
 13. Theaccelerometer as claimed in claim 1 wherein at least one of the planarlayers and the proof mass are formed of different forms of the samematerial.
 14. The accelerometer as claimed in claim 1 wherein the firstsupport structure includes a plurality of beams for suspending the proofmass at upper and lower portions thereof.